Technical Field
The present invention relates to an impedance converter and a condenser microphone using the impedance converter.
Related Art
Among impedance converters, there is one that receives an input voltage at high impedance and outputs the voltage at low impedance, like an impedance converter built in a condenser microphone, for example.
A condenser microphone has small effective capacitance, and thus needs to receive an audio signal from a microphone unit at high input impedance to secure a frequency response up to a low frequency. Further, the microphone is connected to an amplifier through a cable or the like, and thus needs low output impedance when sending an audio output signal of the condenser microphone to the amplifier. Therefore, an impedance converter that converts high input impedance into low output impedance is built in the condenser microphone.
To make characteristics of the output signal of the condenser microphone better, or to make a sound of when the output signal is reproduced more comfortable, there is a condenser microphone using an electron tube as an active element of an impedance converter. The invention described in JP 4227679 B2 is an example using electron tubes as active elements of an impedance converter, and one of embodiments thereof is illustrated in FIG. 3.
In FIG. 3, the impedance converter is mainly formed of two electron tubes 3 and 4 as the active elements. An audio signal is input from an electroacoustic transducer 11 of a condenser microphone module (hereinafter, simply referred to as “module”) 1 to a grid of the electron tube 3 through a capacitor 12. Both the electron tubes 3 and 4 are triodes.
A resistor 35, a plate and cathode of the electron tube 3, a diode 57, and a plate and cathode of the electron tube 4 are connected in series between a positive electrode of a power supply, and a negative electrode, that is, a ground. When a plate current flows in the electron tubes 3 and 4, a voltage caused in the diode 57 is applied to the grid of the electron tube 3 through a diode 55 or a diode 56. The diode 55 and the diode 56 are connected in parallel such that directions into which the current flows are reverse to each other.
The plate of the electron tube 3 is connected to the negative electrode of the power supply through a capacitor 36 and a load resistor 41. A capacitor 34 is connected in parallel to the diode 57. This impedance converter has a cathode follower configuration, and the cathode of the electron tube 3 is connected to an output terminal 9.
The grid voltage of the electron tube 3, which is the grid voltage applied through the two diodes 55 and 56 connected reversely to each other in parallel, converges to a bias voltage of the electron tube 3 as a voltage between terminals of the diode 57, and no current flows in the diodes 55 and 56. Therefore, the diodes 55 and 56 are operated as high resistance elements and can make the input impedance high.
A circuit including the electron tube 4 and the load resistor 41 is operated as a constant current load of the electron tube 3. That is, a grid and the cathode of the electron tube 4 are maintained at the same potential by the load resistor 41 and the electron tube 4 allows a constant plate current to flow, and therefore the circuit including the electron tube 4 and the load resistor 41 serves as the constant current load for the electron tube 3.